The production of structures having high mechanical performances and weights that are as low as possible leads the designers of the structures to use titanium-based metal alloys and composite materials comprising carbon, glass or aramid fibers held in a hardened organic matrix, in general a polymer resin.
When these parts must be drilled or reamed in order to produce assemblies, the drilling conditions must be adapted as a function of the material machined.
Thus, titanium alloys, the thermal conductivity of which is low, around ten times lower than that of an aluminum, are generally drilled using tools having substrates made of tungsten carbide (WC) with cobalt binder that withstand temperatures that may reach 1000° C. during the drilling using an oily lubricant. These high temperatures are the cause of accelerated wear of the drill bits used for the drilling.
On the other hand, composite materials, which have high abrasive characteristics, in particular in the case of carbon fibers, which damage the drilling tool by abrasion, are generally dry drilled with tools made of tungsten carbide with diamond or diamond-coated inserts, preferably polycrystalline diamond (PCD) inserts.
Owing to these very different drilling conditions and to the need to use drill bits specific to each of these conditions, the drilling of a stack of parts made of materials of a titanium alloy and of a carbon fiber composite proves difficult.
In particular, owing to the fact that composite materials, for the most common ones, should not locally exceed a glass transition temperature of the resin, in the case of the most common ones a temperature of the order of 180° C., and that the tools for drilling composite materials are not suitable for drilling titanium alloys due to the temperature reached which would damage the tool, the graphitization of the diamond taking place at around 800° C., the one-step drilling of a stack of these various materials is in general carried out with a tool suitable for drilling titanium while increasing as much as necessary the amount of lubricant in order to limit the increase in temperature.
The drilling of the part of the stack made of composite material is then penalized by an accelerated wear of the drilling tool via abrasion.